Micromechanical devices are used in many applications such as pressure sensing, acceleration sensing, medical diagnostics, surgical systems, and environmental sensing. Originally, micromechanical devices were commonly based on Micro Electro Mechanical Systems (MEMS) technology, in which integrated circuit fabrication techniques are exploited to make micron-scale mechanically active devices and systems. More recently, the field of micromechanics has also come to include nanometer-scale, nanotechnology-based devices and related technology.
Historically, micromechanical device have been made using planar processing techniques, wherein a series of layers of structural material and sacrificial material are deposited and patterned into their desired shapes on a substrate in sequential fashion. By appropriately shaping each successive layer of structural material, a three-dimensional structure can be developed from multiple two-dimensional layers. If an element of the structure is intended to be mechanically active, the structural material of that element is encased in sacrificial material so that it is not locked into contact with other structural material or the underlying substrate.
Once the structure of the micromechanical device is fully formed, the sacrificial material is removed in what is referred to as a “release etch,” which frees the structural material of the mechanically active element by selectively removing the sacrificial material in which it is encased. This renders the element movable with respect to the underlying substrate, as well as other structural material disposed on the substrate.
Because an element must be surrounded by sacrificial material in order to enable its motion after release, it is difficult to fabricate a micromechanical device having a mechanically active element whose unactuated (i.e., quiescent) position leaves it in contact with another structural element on the substrate. Instead, after the release etch, a typical mechanically active element is left separated from other structural elements (or the substrate) by a gap that is substantially equal to the thickness of the sacrificial material removed during the release etch. Only by imparting a force on the mechanically active element (e.g., by means of an actuator) after it has been released can it be brought into contact with the other structural element.
In many applications for micromechanical devices, however, a quiescent state wherein a mechanically active element is in contact with another element is highly desirable. In a microfluidic application, for example, it is often desirable that a flow valve is closed in the absence of power to the device. Or, in a microrelay system, a normally closed device would provide greater system design flexibility and, often, reduced system complexity. A micromechanical device having a normally closed state, therefore, would be an important advance in the state-of-the art.